Cadmium in Soil and Plants

Berkeley, California, 24 June 1997
Edited by M.J. McLaughlin, and B. R. Singh

This symposium was part of the Fourth International Conference on Biogeochemistry of Trace Elements(4th ICBTE), held at the University of California, Berkeley, USA, in June 1997. It was cosponsored by IMPHOS and others, including PPI, IFA, FIFA and Norsk Hydro.

This symposium brought together leading scientists in the field of cadmium reaction in soils and plants, in order to review the current state of knowledge, review the literature and highlight gaps that need to be addressed for effective management of cadmium.

Cadmium transfer through the food chain is an important issue at present. Therefore, it is essential for regulatory agencies attempting to initiate risk reduction strategies, to dispose of reliable scientific data and information for developing appropriate regulations and strategies.

Cadmium may accumulate in crops to levels that are of concern in the diet. Cadmium accumulation in crops is influenced by crop genetics and Cd activity in the soil solution. Therefore, management practices can be used to reduce the level of cadmium in edible portion of crops and its bioavailability to mankind.

Crop selection, based on the progress make with sunflower and durum wheat, it appears that crop selection has great potential for decreasing the concentration of Cd in crops. Crop breeding can also be an important method to limit Cd uptake and accumulation in crops, but the process is long and complex.
Indeed, crop species and cultivars differ widely in their ability to absorb, accumulate and tolerate Cd. Cereals and legumes accumulate less Cd in the shoots than do leafy croops such as lettuce, curlycress and spinach. In general, monocotyledons show lower Cd uptake than dycotyledons.

Phytoremediation is the selection of plants with improved ability to accumulate Cd in order to make it possible to remove significant quantities of Cd from high-Cd soils. On highly polluted sites, use of hyperaccumulator plants which remove large quantities of Cd through accumulation of the element in their above-ground tissues can be effective means to reclaim these sites. Phytoremediation is a very new field, but it holds great potential for remediation of contaminated soils. More research is required to assess the suitability and economic feasibility of such methods.

Plant Nutrient Management practices such as liming of acid soils, use of organic materials low in cadmium to reduce phytoavailability, use of mineral nutrient sources with minimal impact on Cd accumulation, Zn applilcation in Zn deficient conditions to reduce Cd uptake, and avoidance of addition of Cl salts through irrigation should be employed to reduce Cd accumulation.

Dietary Cadmium Exposure differs according to cadmium bioavailability in foods. Research findings under extreme soil contamination conditions demonstrated that no Cd disease occurred in communities with balanced diet.
The food chain has been found to prevent the transfer of soil cadmium in nearly every case, through food or feed chains, causing no human or livestock disease as a general case. The only exception is rice and tobacco grown on contaminated soils. It is now understood that rice grain is seriously deficient in Fe and Zn, and Ca for human needs. Evidence indicates that combined Fe and Zn deficiencies can increase Cd retention by 15-fold compared to Fe and Zn adequate diets.

The proceedings include oral presentations, spread over the following topics :
1. Cadmium in soils and plants
2. The environmental chemistry of cadmium
3. Chemistry of cadmium in soil solution
4. Solid phase of cadmium and reactions of aqueous cadmium within soil surfaces